The invention relates to a special effects device for a musical instrument, such as an electric guitar, having an electrical output.
1. Field of the Invention
The present invention relates generally to apparatus for producing unusual audio effects. In particular, the invention relates to a device that produces a complete and actual signal reversal after an initial time delay that is equal to the duration of the first initial inputted signal. In a second mode, the device will produce an accurate simulated signal reversal, with no apparent time delay, by storing the attack dynamic and an initial portion of the signal decay, while modulating the signal gain to simulate the amplitude envelope of a reversed signal. The stored portion of the signal is then xe2x80x9csplicedxe2x80x9d onto the simulated reversed signal part way through its duration, ideally by matching the envelope height and the beginning of the repetitive waveform that comprises the note being reversed.
2. Background
A musical note, or tone, produced by a musical instrument of string, piano or percussion type, and, in particular, electric and electronic guitars, is characterized by a repetitive waveform of relatively high frequency within an envelope of much lower frequency having a relatively rapid rise (corresponding to the plucking of the guitar string) and a relatively slow decay as the string releases its energy in the form of a musical note. This relatively slow decay is usually exponential in nature.
If a single note is recorded and then played backward, the played back backward (or xe2x80x9creversedxe2x80x9d) note will be characterized by a reversed repetitive waveform of relatively high frequency and an envelope having a relatively slow exponential rise and a relatively fast decay. Producing a series of notes where the notes are played back in the same sequence but each single note is played xe2x80x9cbackwardxe2x80x9d, as if time reversed, and has a slow exponential rise and fast decay produces an unusual and pleasing effect that is useful in the recording of modern music. It is especially effective for electric and electronic instruments such as the electric guitar.
The typical method of generating a note backwards has been to record the note on magnetic or a vinyl medium as it would normally be heard and to then reverse the direction of play of the medium. The drawbacks of this awkward, cumbersome method are that it requires a medium having prerecorded tones or notes disposed on it and cannot be used in live performance; also the medium is often damaged when the direction of play is reversed.
Prior art technology produced such an effect. The prior art technology also accomplished the entering of a musical note into a device such as a random access memory chip and playing the note backwards and simultaneously entering the next note into memory. This was done by filling and emptying the memory.
Prior art note reversal devices also utilized predetermined or fixed address limits or packet lengths. The playback of the decay of a note as the attack of the reversed note was of a fixed length regardless of the length of the original decay.
Input signals were often broken up into segments if the reversed note exceeded the length of the packet. As can be understood from the above, the prior art technologies necessarily result in at least a one note delay and involve truncations of the reversed note playback in the event of notes of varying length, due to the use of fixed packet lengths.
It is an object of the present invention to provide a simple method of generating musical tones or notes in reverse without utilizing magnetic recording technologies. It is also an object of the present invention to provide a music signal converter device for producing an output signal, which for each note signal, is either a substantially faithful time reversed version of the input note signal, particularly with regard to envelope shape and duration and saved attack component, or a complete and actual reversal of the input note. It is another object of the present invention to address signal delays and accommodate notes of varying length.
It is a further object of the present invention to provide a realistic and highly accurate truer real time reverse tape simulation by utilizing a generated envelope based upon the inverse normalized slope of the decay envelope of the sampled inputted note and mating it with the actual attack dynamic which is saved in memory, reversed and spliced onto the xe2x80x9csimulatedxe2x80x9d envelope.
It is a further object of the present invention to eliminate the dependency of the attack of the reversed note on a fixed length of time. The present invention utilizes address limits based on the note envelope passing through amplitude thresholds so that both the beginning and the end of the note produced are determined from the characteristics of the input note. For a given note, the duration of the output of the inventive system corresponding to the playback of the note in reverse, corresponds to the amount of space in memory required to store the note. Hence, the time period of the output note is variable and dependent upon the duration of the input note.
These objects of the invention in the embodiment of a true note reversal system are achieved in accordance with the preferred embodiments by a method for reversing a series of notes, in which each of the notes have a waveform, comprising the steps of digitizing sequential, storing the digitized notes in memory, and playing back the digitized notes, the playing back being done with the waveform of each note reversed but with the notes in the original sequence of the series, playback of a reversed note beginning after the end of the note is detected and the entire note has been stored in memory. The notes are not broken up, recorded and played back in fixed length packets, nor is the filing/emptying of memory based upon specific time periods or determined by predetermined fixed memory address limits, and the duration of the recording and playback notes to and from memory is made variable and a function of the actual length of the notes. The amount of memory occupied by an individual note varies with the length of the note, and silence intervals between successive notes are encoded during recording and reproduced during playback, the silence intervals not being stored in memory. The beginning of a note is detected by the input audio signal exceeding a variable amplitude threshold for a predetermined period of time. A end of a note is determined by the input audio signal not exceeding a variable amplitude threshold for a predetermined time interval, the variable threshold being derived proportional to the peak attack amplitude of the note, whereby softer notes are prevented from being terminated prematurely as compared to the operation of a fixed end note threshold. Addressing of memory used to store digitized notes is controlled by a microcontroller with memory organized in segments, the microcontroller being linked in software, to maximize the efficient utilization of memory. Then start of a note is detected without clipping, audio is continuously recorded in one memory segment; when the end of this segment is reached, recording wraps back to the beginning so as to continuously loop through the segment; when the beginning of a note is detected, the start address of the note is stored and recording then advances to reverse playback of the note, when the last memory segment is reached, playback begins at the start address and decrements through the segment, for a fixed time interval, to assure that the entire note attack is played in reverse, including the portion which may have occurred prior to detection of the beginning of the note.
The inventive method for the simulation of a realistic reverse tape effect, in real time, comprising the steps of detecting the beginning of a note by monitoring the input audio signal to determine when the same exceeds a variable amplitude threshold for a predetermined period of time and initially modulating audio signal gain with a rising exponential, while simultaneously recording the note attack and initial portion of the note decay period in digital memory, and when the input note amplitude envelope has decayed to a predetermined percentage of its peak value, stopping recording and beginning playback of the previously stored portion of the note decay and attack dynamic. The inflection point amplitude envelope, i.e. the boundary between the note attack and decay portions of the note envelope is detected to determine the peak amplitude of the note waveform, the detection being done by software calculation and monitoring for decreasing successive peak signal amplitudes at between 6 and 26 millisecond intervals. The inflection point amplitude is used to digitally normalize the amplitude envelope of a note by dividing down the peak amplitude, by powers of two, and combining the quotients to create fractional amplitude thresholds; the decay portion of the input amplitude envelope is compared against the thresholds to give a measure of the rate fo decay or slope of the exponential note envelope. As a note decays through each calculated amplitude threshold, microcontroller software modulates the gain of a multiplying digital to analog converter from an initial value of substantially zero to a maximum predetermined gain, to create a rising exponential gain transfer function. A rising exponential gain transfer function is created by varying the reference voltage of the multiplying digital to analog converter through a pulse-width gain modulator. When a note envelope decays to between ten and thirty percent of its peak amplitude, the decaying exponential and its inverse (mirror image) intersect, the D/A converter gain is increased to its maximum value of gain, and the previously recorded portion of the note decay and attack dynamic are xe2x80x9csplicedxe2x80x9d into the simulated reverse note effect by playing them back in reverse from memory. During reverse playback from memory, the microcontroller monitors the analog to digital converter signal data to detect premature termination of a note decay, such as that caused by a musician muting the guitar string, or the start of a new note to terminate playback, the playback being otherwise terminated when the note start address is reached and a pre-start audio interval is played and recording is then advanced to reverse playback of the note, when the last memory segment is reached, playback begins at the start address.